We are studying activity-dependent forms of synaptic modification that are important in the representation of stimuli in the CNS during learning. It has long been hypothesized that the dually-regulated, calcium/calmodulin-sensitive adenylyl cyclase enzyme plays a role in associative learning in the marine snail Aplysia, in fruit flies and in mammals. Although this enzyme has been shown to be critical in learning in both Drosophila and mice, it has been difficult to directly explore the hypothesis that calmodulin-sensitive adenylyl cyclase serves as a molecular coincidence detector, integrating the cellular signals Ca and modulator/ neurotransmitter. We have now cloned three isoforms of adenylyl cyclase in Aplysia, including one that binds to and is stimulated by calmodulin. In cellular electrophysiological experiments we will test the role of convergent activation of this adenylyl cyclase in associative synaptic strengthening during conditioning. During conditioning, the associative synaptic modification enhances the importance of particular stimuli for the animal. Non-associative activity-dependent plasticity also plays an essential role in learning, determining which stimuli receive effective attention. This is critical in the context of learning, because without appropriate afferent transmission of information about stimuli, learning is not possible. In the activation of Hebbian mechanisms of synaptic strengthening during learning, it is essential that afferent input successfully activate the postsynaptic neurons. We are analyzing an activity-dependent switch that can either shut off or maintain synaptic transmission in a sensory pathway, depending on the precise pattern of sensory neuron activity. This switch involves activity-dependent, calcium-mediated activation of protein kinase C, precisely localized to presynaptic transmitter release sites. The pathway that is regulated in an activity-dependent manner appears to involve phospholipid synthesis and the small GTPase ARF. We will explore the molecular steps in this pathway, using RNAi, dominant-negative proteins and caged lipids. These studies are relevant for both for learning and for attention disorders.